1. Field of the Invention
The present invention relates to communications systems that transmit signals that may be composed of plural combined transmit carriers. Each of these carriers may include one or more communication channels. More particularly, the present invention relates to wireless communications systems and signal processing apparatus employed in wireless communications systems. The term ‘wireless communications systems’ includes cellular communication systems, personal communication systems (PCS), wireless local loop systems, and all other like systems.
2. Background of the Prior Art and Related Information
Wireless communications systems employing transmission between base stations and remote users are a key component of the modern communications infrastructure. These communication systems are being placed under increasing performance demands that are taxing the capability of available equipment, especially wireless base station equipment. These increasing performance demands are due to both the increasing numbers of users within a given wireless region, as well as the bandwidth requirements allocated to wireless system service providers. The increasing number of wireless users is of course readily apparent and this trend is unlikely to slow due to the convenience of wireless services. The second consideration is largely due to the increased types of functionality provided by wireless systems, such as wireless Internet access and other forms of wireless data transfer over such systems. These considerations have resulted in a need for more carriers operating from each transmitting location of a wireless service network.
There are several methods for creating multiple communication channels on a single carrier. These methods include code division multiple access (CDMA), time division multiple access (TDMA), as well as others. In each of these methods, several data channels enter a signal generator that combines the input data channels using one or more of the methods mentioned above to produce a complex signal output. FIG. 1 shows M multiple channel signal generators 1 each producing a complex pair of signal outputs labeled Vm,1 and Vm,2. Each of these complex pairs is then filtered 2, frequency converted 7, and combined 8 to produce a single complex pair. This data pair is then D/A converted 3 to a baseband analog signal, modulated 4 to RF, amplified 5 and transmitted with an antenna 6.
When generating a multiple carrier signal as shown in FIG. 1, the output signal peak-to-average ratio is determined by the signal generators 1, the filters 2, frequency offset 7, and the interaction of each carrier in combining 8. The combined multiple carrier signal must then be D/A converted, modulated to RF, and amplified. High peak-to-average ratios require increased cost in D/A converter 3, RF up converting modulator 4, and power amplifier 5 components. D/A converters with large bit counts must be used to both create the large peaks and maintain adequate signal resolution to overcome noise generated in the D/A conversion process. High peak-to-average ratios require the use of very linear RF up converting modulator and power amplifier components to prevent signal distortion from increasing carrier bandwidth through distortion and intermodulation. Signal bandwidth is government regulated. Increased carrier bandwidth may cause operation outside government allocated operating bands in violation of the law. Therefore, a need presently exists for a communications system that reduces or eliminates high peak-to-average ratios.